<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        div{
            width: 100px;
            height: 100px;
            background: red;
        }
    </style>
</head>
<body>
    <div id="tes" class="test"></div>
    <script>
        var test = document.getElementById("test")||document.getElementsByClassName("test")[0];
        console.log(test);
        test.onclick = function(){
            this.style.backgroundColor = "green";
        }
        /*&&,||,!
        结果返回boolean值
        && 两边都为true，结果为true
        || 只要一边为true，结果就为true
        ！ 取反*/
        var a = 10;
        var b = 20;
        var c = 30;
        var d = 40;
        console.log(a<b && c>d);
        console.log(a<b && c<d);
        console.log(a<b || c>d);    
        console.log(""||"abc");
        // 两边都是字符串所以将它们都先转换成boolean值
        // ""-->false "abc"-->true
        // 结果为abc
        function b(){

        }
        console.log(typeof b);
        // 返回的b为函数类型  function
    </script>
</body>
</html>